This invention relates to non-destructive testing methods and apparatus for identifying types of intrinsic flaws in metallic sputter target materials and, more particularly, non-destructive methods and apparatus for identifying and counting of solid inclusions using radio frequency echo waveform phase change detection.
Cathodic sputtering is widely used for depositing thin layers or films of materials from sputter targets onto desired substrates such as semiconductor wafers. Basically, a cathode assembly including a sputter target is placed together with an anode in a chamber filled with an inert gas, preferably argon. The desired substrate is positioned in the chamber near the anode with a receiving surface oriented normal to a path between the cathode assembly and the anode. A high voltage electric field is applied across the cathode assembly and the anode.
Electrons ejected from the cathode assembly ionize the inert gas. The electrical field then propels positively charged ions of the inert gas against a sputtering surface of the sputter target. Material dislodged from the sputter target by the ion bombardment traverses the chamber and deposits on the receiving surface of the substrate to form the thin layer or film.
One factor affecting the quality of the layer or film produced by a sputtering process is the xe2x80x9ccleanlinessxe2x80x9d of the material from which the sputter target is made. The term xe2x80x9ccleanlinessxe2x80x9d is widely used in the semiconductor industry, among others, to characterize high purity and ultra high purity materials. In common practice, xe2x80x9ccleanlinessxe2x80x9d refers to the degree of material internal purity. Such impurities may be present, for example, as traces of foreign elements in distributed or localized form in the sputter target material. Cleanliness is usually measured in units of particles per million (xe2x80x9cppmxe2x80x9d) or particles per billion (xe2x80x9cppbxe2x80x9d) which define a ratio between the number of contaminant atoms and the total number of atoms sampled.
Since the cleanliness of the material from which a sputter target is made affects the quality of layers of films produced using that target, it is obviously desirable to use relatively clean materials in fabricating sputter targets. This implies a need in the art for non-destructive techniques for selecting sputter target blanks of suitable cleanliness to produce high quality sputter targets. Known destructive test methods, such as glow discharge mass spectroscopy and LECO techniques, are not suitable for this purpose.
Another factor affecting the quality of the layer or film produced by a sputtering process is the presence of xe2x80x9cflawsxe2x80x9d in the sputter target material. As used herein, the term xe2x80x9cflawsxe2x80x9d refers to microscopic volumetric defects in the sputter target material, such as inclusions, pores, cavities and micro-laminations. However, not all the flaws are xe2x80x9calikexe2x80x9d in their degrading effect on sputter performance. Some types of flaws, for example, micro-cavities or shrinkage porosity cause relatively xe2x80x9cmildxe2x80x9d degrading effect on sputter performance while the others, such as dielectric inclusions, cause a serious disturbance in the sputter process. Therefore, there exists a corresponding need in the art for a non-destructive technique which identifies and separately counts different kinds of flaws which may exist in sputter target materials.
FIG. 1 illustrates a prior art non-destructive ultrasonic xe2x80x9cflawxe2x80x9d detection method for characterizing aluminum and aluminum alloy sputter target materials. The technique illustrated in FIG. 1 is similar to that suggested in Aluminum Pechiney PCT Application No. PCT/FR96/01959 for use in classifying aluminum or aluminum alloy blanks suitable for fabricating sputter targets based on the size and number of internal xe2x80x9cdecohesionsxe2x80x9d detected per unit volume of the blanks.
The prior art technique of FIG. 1 employed a pulse-echo method performed on a test sample 10 having a planar upper surface 12 and a parallel planar lower surface 14. In accordance with this technique, a focused ultrasonic transducer 16 irradiated a sequence of positions on the upper surface 12 of the test sample 10 with a single, short-duration, high-frequency ultrasound pulse 18 having a frequency of at least 5 MHz, and preferably 10-50 MHz. The ultrasonic transducer 16 then switched to a sensing mode and detected a series of echoes 20 induced by the ultrasound pulse 18.
One factor contributing to these echoes 20 was scattering of sonic energy from the ultrasound pulse 18 by flaws 22 in the test sample 10. By comparing the amplitudes of echoes induced in the test sample 10 with the amplitudes of echoes induced in reference samples (not shown) having compositions similar to that of the test sample 10 and blind, flat-bottomed holes of fixed depth and diameter, it was possible to detect and count flaws 22 in the test sample 10.
The number of flaws detected by the technique of FIG. 1 had to be normalized in order to facilitate comparison between test samples of different size and geometry. Conventionally, the number of flaws was normalized by volumexe2x80x94that is, the sputter target materials were characterized in units of xe2x80x9cflaws per cubic centimeter.xe2x80x9d The volume associated with the echoes 20 from each irradiation of the test sample 10 was determined, in part, by estimating an effective cross-section of the pulse 18 in the test sample 10.
A portion of the scattered energy is attenuated by the material making up the test sample 10. Furthermore, since the single flaw sizes of interest, which range from approximately 0.04 mm to 0.8 mm, are of same range with the wavelength of ultrasound in metals (for example, the wavelength of sound in aluminum for the frequency range of 10 MHz to 50 MHz is 0.6 mm to 0.12 mm respectively), the pulse 18 has a tendency to refract around the flaws 22, which reduces the scattering intensity.
Another factor detracting from the ability of the transducer 16 to detect the sonic energy scattered by the flaws 22 is the physical nature of the substance of the flaw or more accurately a degree of acoustic impedance mismatch at the flawxe2x80x94matrix material boundary. The impedance mismatch directly affects the reflection and transmission characteristics of ultrasound at the phase boundaries. The reflection coefficient of ultrasound beam at matrix-to-flaw boundary can be expressed by the simplified expression: R=(I2-xe2x88x92I1)/(I2+I1), where I2 is an acoustic impedance of the flaw material, and I, is an acoustic impedance of the matrix material. The simple analysis of this formula allows us to derive several important conclusions. At first, if acoustic impedance of the flaw I2 is less than the acoustic impedance of the matrix I1, then the R coefficient becomes negative. The negativity of the R can be translated as a change in the phase of the acoustic pulse waveform on 180xc2x0. For example, if the flaw is the gas-filled or vacuumed (shrinkage) void with the acoustic impedance equal to 0.93 g/cm2-sec(xc3x97106) (air) or below (vacuum), then the phase of the ultrasound pulse waveform is changed on 180xc2x0 at the boundary. At second, if the flaw is a gas filled or vacuumed void in the aluminum matrix with the acoustic impedance of 17.2 g/cm2-sec(xc3x97106), then the reflection coefficient value is close to the unity or 100% and the amplitude of the reflected signal is the only function of the relationship between flaw size and the ultrasound beam focal spot size. At third, if the flaw comprises a solid particle, for example, an alumina inclusion with the acoustic impedance of 39.6 g/cm2-sec(xc3x97106), which exceeds the acoustic impedance of the aluminum matrix more than two times (17.2 g/cm2 sec(xc3x97106)), the ultrasound waveform does not experience the phase inversion at the flaw boundary, and for alumina inclusion the reflection coefficient does not exceed 39.5% of the amplitude of the impinging pulse (if the wave interference effect is not considered). In this case, the amplitude of the reflected signal is the function of two variables, firstly, the relationship between flaw size and the beam focal spot size, and secondly, the degree of acoustic impedance mismatch at flaw-to-matrix boundary.
Therefore, the final conclusion is that the void-like flaw and alumina inclusion of same size reflect the ultrasound energy quite differently. In addition to the waveform phase inversion, the amplitude of the reflected signal from the void-like flaw is at least two times higher than for the alumina particle inclusion. Hence, the detectability of alumina inclusions is generally poorer than the detectability of void-like flaws, and if the phase information for reflected signal is not extracted simultaneously with the amplitude information, the testing results can be misleading caused by misinterpretation of the actual larger alumina particle with the smaller void-like flaw and vice versa.
Another factor detracting from the ability of the transducer 16 to detect the sonic energy scattered by the flaws 22 is the noise generated by scattering of the pulse 18 at the boundaries between grains having different textures. In fact, the texture-related noise can be so great for high-purity aluminum having grain sizes on the order of several millimeters that small flaws within a size range of approximately 0.05 mm and less cannot be detected. Larger grain sizes reduce the signal-to-noise ratio for the sonic energy scattered by the flaws when compared to the noise induced by the grain boundaries.
Other factors affecting the sensitivity and resolution of the technique of FIG. 1 include the pulse frequency, duration and waveform; the degree of beam focus and the focal spot size; the coupling conditions (that is, the efficiency with which the sonic energy travels from the transducer 16 to the test sample 10); and the data acquisition system parameters.
One major drawback to the technique of FIG. 1 is an inability of the technique to distinguish between different sorts of flaws, particularly between void-like flaws and solid particle inclusions, such as alumina particles. This technique, which relies only on the echo amplitude measurements, confirms only the physical existence of the flaw. Its physical nature and actual size are not properly revealed and derived only on the basis of the flaw type assumption. If the internal xe2x80x9cdecohesionsxe2x80x9d (void-like defects) are the only defects in the target material, then the technique as referred in the method (FIG. 1) is able to detect and size defects adequately. However, in reality the internal xe2x80x9cdecohesionsxe2x80x9d as referred in the method (FIG. 1), are the fraction of plurality of defect types which may exist in the target material. For example, the metallographic evaluation revealed also aluminum oxide particles in the aluminum for sputter targets. Therefore, the technique as referred in the method (FIG. 1) is unable to distinguish and to differentiate between pluralities of flaw types since the waveform phase change information remains not revealed.
Thus, there remains a need in the art for non-destructive techniques for characterizing sputter target materials having different pluralities of flaw types. There also remains a need for a technique that separately compares the target intrinsic volumetric cleanliness for the specific groups of flaws such as void-like flaws (cavities, microlaminations, xe2x80x9cdecohesionsxe2x80x9d) and solid inclusions.
One imaging technique implemented by Sonix, Inc. (8700 Morrisette Dr., Springfield, Va. 22152) in a FlexSCAN-C C-scanning uses a phase gating method which detects the phase inversion in the waveform at the matrix-to-flaw boundary. The technique uses a xe2x80x9cTexas Instrumentsxe2x80x9d phase inversion algorithm (licensed to SONIX, Inc.). The technique maps the flaws on a two-dimensional sample image only if the 180xc2x0 phase change is detected. Therefore, this technique is limited to detection and mapping void-like defects when the impedance is changed from higher to lower at the flaw boundary. For sputter target applications however, it is absolutely necessary to detect and identify the alumina particle-inclusions, and the phase inversion technique used by the Sonix. Inc. does not work in this case since the waveform does not change its phase at the flaw boundary.
There also remains a need for a technique that separately detects and sizes specific alumina particle-inclusions.
These needs and others are addressed by a non-destructive method for characterizing a sputter target material comprising the steps of sequentially irradiating a test sample of the sputter target material with sonic energy at a plurality of positions on a surface of the sample; detecting echoes induced by the sonic energy; discriminating texture-related backscattering noise from the echoes to obtain non-rectified radio frequency echo waveform signals; monitoring non-rectified echo waveform signals for the 180xc2x0 waveform phase inversion, comparing the non-rectified echo waveform signals with said at least one of each: phase inverting and phase non-inverting reference values, to detect void-like and particle-like flaw data points separately and no-flaw data points; counting the flaw data points for the each flaw type separately as well as all together to determine a total flaw count CFT(TOTAL); CFI(with phase inversion), flaw count without phase inversion CFN(without phase inversion), counting the flaw data points and the no-flaw data points to determine a total number of data points CDP and calculating a total cleanliness factor FCT=(CFT/CDP)xc3x97106 as well as cleanliness factors FCI=(CFI/CDP)xc3x97106 and FC=(CFN/CDP)xc3x97106 for each sort of flaws separately.
Unlike the prior art method described earlier, the method of the present invention provides a characterization of the sputter target material by separately identifying and counting void-like and particle-like flaws. A partition of cleanliness factor for components associated with different kinds of flaws tunes up the rejection criteria more precisely by identifying and sizing the flaws of different kind.
Unlike the Sonix, Inc. method, the method of the present invention provides a characterization of both the waveform phase inverting and phase non-inverting flaws. Therefore, there is a smaller risk to miss the waveform phase non-inverting flaws which are of a primary concern for sputter target applications.
Although the cleanliness factor technique provides a useful characterization of the sputter target material, more information can be provided by means of a histogram. More specifically, the sputter target test method may be characterized by defining a plurality of amplitude bands for each type (waveform inverting and non-inverting) of flaws; measuring said modified amplitude signals to determine modified amplitude signal magnitudes; comparing said modified amplitude signal magnitudes with said plurality of amplitude bands to form subsets of said modified amplitude signals; counting said subsets of modified amplitude signals to determine a plurality of modified amplitude signal counts, each modified amplitude signal count of said plurality of amplitude signal counts corresponding to one of said amplitude bands of said plurality of amplitude bands; and constructing a pareto histogram, combining individual histograms for both flaw classes, relating said modified signals counts to said plurality of amplitude bands. Since the histogram does not attempt to directly map the locations of flaws along the surface of the sputter target material, it does not suffer from the scaling problems.
Most preferably, the test sample is compressed along one dimension, such as by rolling or forging, and then irradiated by sonic energy propagating transversely (that is, obliquely or, better yet, normally) to that dimension. This has the additional effect of flattening and widening of certain flaws (aluminum oxide film clusters and voids) in the material. The widening of the flaws, in turn, increases the intensity of the sonic energy scattered by the flaws and reduces the likelihood that the sonic energy will refract around the flaws.
These methods for characterizing sputter target materials may be used in processes for manufacturing sputter targets. As noted earlier, the cleanliness of a sputter target and particularly cleanliness from non-phase inverting flaws is the primary factor determining the quality of the layers or films produced by the target. By shaping only those sputter target blanks having cleanliness factors or histograms meeting certain reference criteria to form sputter targets, and rejecting blanks not meeting those criteria, one improves the likelihood that the sputter targets so manufactured will produce high quality layers or films.